Your best graphics card bet is probably the GTX 750ti. You will however, definetly want to upgrade your ram. I would seriously consider getting another 4GB of RAM for a total of 8. With this combo, you will be playing most games at low to medium settings with +30 fps. You can find the 750ti at a variety of places, so it is your choice as to which one to get.
■ A cheap, good quality one is this Zotac one.
■ A more expensive, high quality one is this EVGA Superclocked one
If you have any questions about them, feel free to private message me or put it in this thread and i'll let you know!